Lane width is one of several important variables of the cross-sectional road design. Cross sectional elements such as lane and shoulder width vary depending on roadway function, traffic volume, and design speed. According to the geometric design manuals, higher traffic volume and design speed require wider lanes and shoulders.
The findings or processes concerning lane width that are transferable to or operative in all countries are that narrow lanes invite lower speeds and have a higher risk of same or opposite direction contact between vehicles, and that wider lanes invite higher speeds and a lower risk of same or opposite direction contact between vehicles. Also a more general finding corroborated in various studies is that on two-lane rural or urban roads the widening of lanes tends to improve road safety.
For rural two-lane highway roads there is robust evidence that increasing lane width reduces the occurrence of single vehicle run-off-road same and opposite direction crashes. However, at the same time studies have indicated that very wide lanes (or shoulders) may increase crash risk mainly due to higher speeds. American experts have agreed upon crash modification factors for lane and shoulder widths in the Highway Safety Manual. Some studies in the USA show road safety benefits by decreasing lane width whereas others show negative effects. For roads with 3.65 metre lanes and with speed limits of 40 mph (around 65 km/h) or higher it was estimated that decreasing lane width would produce better safety results than increasing lane width. Other studies in the USA show that adding a lane by narrowing existing lanes on four-lane urban freeways may increase accidents. These conflicting results illustrate the importance of looking at combinations of cross-sectional elements rather than trying to isolate one specific element.
For those roads in Europe which are similar to the studied American roads in terms of main design variables such as speed limit, lane width, number of lanes, shoulder width, traffic volume and lane separation, the results from American research may be used as a valid reference.